1. Field of the Invention
The present invention relates to an organometallic complex for an organic electroluminescent device, and more particularly to an organometallic complex serving as a light-emitting layer of an organic electroluminescent device.
2. Description of the Related Art
An organic electroluminescent device (also referred to as organic light-emitting diode; OLED) is an LED with an organic layer serving as the active layer, increasingly applied in flat panel displays due to advantages, such as low voltage operation, high brightness, light weight, slim profile, wide viewing angle, and highly effective contrast ratio.
Generally, an OLED is composed of a light-emitting layer sandwiched by a pair of electrodes. Light emission is caused by the following phenomenon. When an electric field is applied to these two electrodes, the cathode injects electrons into the light-emitting layer and the anode injects holes into the light-emitting layer. When the electrons recombine with the holes in the light-emitting layer and excitons are formed. Recombination of electron and hole is accompanied by emission.
Depending on the spin states of the hole and electron, the exciton which results from hole and electron recombination can have either a triplet or singlet spin state. Luminescence from a singlet exciton results in fluorescence whereas luminescence from a triplet exciton results in phosphorescence. The emissive efficiency of phosphorescence is three times that of fluorescence. Therefore, it is crucial to develop highly efficient phosphorescent material, in order to increase the emissive efficiency of the OLED.
Certain organometallic complexes have been reported as having intense phosphorescence (Lamansky, et al., Inorganic Chemistry, 2001, 40, 1704), and efficient OLEDs emitting in the green to red spectrum have been prepared with these complexes (Lamansky, et al., J. Am. Chem. Soc., 2001, 123, 4304). U.S. Patent Application Publication 2003/0072964A1 discloses a phosphorescent organometallic complex including phenylquinolinato ligands. Other emissive organometallic complexes can be found in U.S. Patent Application Publication 20020024293A1, 20020182441A1 and 20030116788A1.
Relatively good green or red light-emitting organometallic complexes have been developed. More effort in development of organometallic complexes emitting blue light, however, is required.